Fetal liver transplants (FLT) were carried out in 25 beagles under various conditions. Graft recipients were prepared with fractionated total body x-irradiation (TBI) of 3 X 6 Gy or 2 X 6 Gy and rescued with cryopreserved fetal liver cells (FLC) from 51- to 52-day-old or 43- to 46-day-old, DLA-identical siblings or DLA-haploidentical, homozygous half-siblings. In all groups, FLC grafts contained comparable numbers of granulocyte-macrophage progenitor cells. Initial engraftment was achieved in all dogs. However, low TBI dose and DLA haplotype disparity between donor and recipient were associated with graft failure in 1/3 and 2/9 recipients, respectively, within 10-16 days of treatment. Lectin-responsive host type lymphocytes circulated for more than 5 weeks, whereas bone marrow metaphases were always of donor sex. Reduced TBI dose and young donor age were associated with delayed granulocyte recovery. Moreover, circulating platelets and total lymphocytes as well as T and B-cell numbers and rose more slowly in recipients of immature FLC grafts than in the other groups. Delayed cutaneous hypersensitivity reactions were normal one year after FLT, whereas the IgM component of the hemagglutinin response to sheep red blood cells was depressed. In mixed leukocyte culture, chimeric lymphocytes were tolerant to host antigens. Nonetheless, clinical and histological signs compatible with low-grade graft-versus-host disease were recorded in 10 or 25 animals. Thus FLT in dogs could be carried out, even across DLA barriers, without severe graft-versus-host disease. However, a low pretransplant TBI dose, incomplete DLA match and young age of the fetal donor were associated with graft rejection and protracted restoration of hemopoiesis and immune functions.